1. Field of the Invention
The present invention relates to a polishing apparatus, and more particularly to a polishing apparatus for polishing a workpiece such as a semiconductor wafer or the like to a flat mirror finish.
2. Description of the Prior Art
As semiconductor devices become more highly integrated in recent years, circuit interconnections become thinner and the distances between those interconnections also become smaller. Photolithographic processes for producing interconnections that are 0.5 .mu.m wide or smaller, particularly, require a flat image-focusing plane for the stepper because the depth between focal points is small.
Therefore, it is necessary to make the surface of a semiconductor wafer flat before fine circuit interconnections are formed thereon. According to one customary process, semiconductor wafers are polished to a flat finish by a polishing apparatus.
One conventional polishing apparatus comprises a turntable with an abrasive cloth attached to its upper surface and a top ring disposed in confronting relationship to the upper surface of the turntable, the turntable and the top ring being rotatable at respective independent speeds. The top ring is pressed against the turntable to impart a certain pressure to a workpiece which is interposed between the abrasive cloth and the top ring. While an abrasive liquid containing abrasive material is supplied onto the upper surface of the abrasive cloth, the surface of the workpiece is polished to a flat mirror finish by the abrasive cloth which has the abrasive material thereon, during relative rotation of the top ring and the turntable.
To replace the abrasive cloth on the turntable, the polishing apparatus is shut down, and the abrasive cloth is detached from the turntable. Then, any abrasive liquid that remains on the upper surface of the turntable is washed away. After the turntable is dried, a new abrasive cloth is attached directly to the upper surface of the turntable. The above replacement process is not efficient as it is tedious and time-consuming. Because the downtime of the polishing apparatus is long due to the abrasive cloth replacement, the number of production lots per unit time yielded by the polishing apparatus is small.
One solution to the above problems has been to use a cloth cartridge having an abrasive cloth attached to a base, the cloth cartridge being detachably mounted on the turntable. The abrasive cloth can be replaced by replacing the existing cartridge with a new cartridge having a fresh abrasive cloth. Use of such a replaceable cloth cartridge is effective in shortening the cloth replacement process and hence the downtime of the polishing apparatus, so that the number of production lots per unit time yielded by the polishing apparatus can be increased. The cloth replacement process using the cloth cartridge is disclosed in U.S. Pat. No. 4,527,358, for example.
There have been known various mechanisms by which the turntable and the cloth cartridge are detachably fixed to each other. For example, Japanese patent publication No. 59-44185 discloses an arrangement in which the outer circumferential edge of the cloth cartridge is fastened by bolts to the turntable through a holder ring. Japanese laid-open patent publication No. 4-206929 discloses another structure which has interfitting members disposed in corresponding positions on the cloth cartridge and the turntable.
The former mechanism allows the outer circumferential edge of the cloth cartridge to be securely fastened by bolts to the turntable, but is disadvantageous in that replacing the cloth cartridge is tedious and time-consuming because it is necessary to loosen and re-tighten the bolts. With the latter structure, the cloth cartridge can easily be mounted on and dismounted from the turntable by the interfitting members, which are however subject to a certain degree of play. Particularly, when a localized load is imparted from the top ring to the cloth cartridge during a polishing process, the cloth cartridge tends to be displaced, adversely affecting a smooth polishing action and the flatness of the workpiece that is being polished.